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8/17/2019 Construction Techniques for Segmental Concrete Bridges
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Construction Techniques for
Segm ental Concrete Bridges
•2 •
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•Z •
James M. Barker
Associate
H.
W. Lochner Inc.
Consulting Engineers
Chicago Illinois
O
ne of the primary advantages of
segmental concrete bridge con-
struction is the economics. In a large
majority of cases, segmental construc-
tion has been the winner wh ere alternate
construction methods have been avail-
able to contractors at the time of bidding.
There are many reasons for this rela-
tively new method of bridge construction
in the United States com peting so well in
the 10 years since the E uropean transfer
technology was started. I believe, how-
ever, that the principle reason for the
success of segmental concrete con-
struction is the number of construction
techniques available to build these
bridges. Of approximately 35 to 40 such
bridges either completed, under con-
NOTE
This article is based on a presentation given
at the Long Span Concrete Bridge Conference in
Hartford, Connecticut, March 18-19, 1980. The
conference was sponsored by the Federal Highway
Ad ministration, P ortland C ement A ssociation, Pre-
stressed Concrete Institute, Post-Tensioning Insti-
tute, and Concrete Reinforcing Steel Institute.
struction or in design in the United
States, few have been or will be con-
structed in exactly the same manner.
The multitude of choices available to
contractors allows them to tailor each
project to their manpower and equip-
ment in the interest of maximizing effi-
ciency and op timizing cost.
Segmental bridge construction is also
revising the basic thinking of design en-
gineers. Until recently, designers have
concerned themselves mainly on how to
build the project after preparing compu-
tations and plans. Segmental construc-
tion has revised this thinking. The first
question asked about a project now is
What is the best and most economical
way
to build this project? Once this
question has been satisfactorily an-
swered, the designer can proceed with a
design based on the most efficient con-
struction method.
In order to make intelligent decisions,
both designers and contractors need to
become familiar with the available
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IT
RRI G
̂ ̂ ô
o O
Fig. 1. Schematic of short line match casting system. The usual rate of
production is four segments per week per set of forms.
methods of segmental construction. Not
all methods are applicable to all projects
but all should be considered for each
job. There are several fundamental con-
cepts relating to casting and erection of
segments which could be considered for
practically every bridge project. By no
means should these basic concepts be
the only ones considered. Segmental
bridge construction was born out of in-
novation and will continue to grow
through more innovation by both con-
tractors and d esign engineers.
Precasting Tech niques
Short line system
—All
he casting
methods to be discussed utilize the con-
cept of match casting. The basic prem-
ise of match casting is to cast the seg-
ments so their relative erected position
is identical to their relative casting posi-
tion. This requires a perfect fit between
the ends of the segments and is accom-
plished by casting each segment directly
against the face of the preceding one
using a debonder to prevent bonding of
the concrete. The segments are then
erected in the sam e sequence they we re
cast.
The most common method for match
casting segments is called the short
line method. Fig. 1 shows a schematic
of a short line match casting system.
PCI JOURNAUJuly-August 1980
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Fig. 2. Casting machines (forms) fold back hydraulically or with
screw jacks to permit moving of segments.
With this system, the rate of segment
production will approach one segment
per line of forms per day. A good aver-
age to use for a project is four segments
per line ev ery 5 days.
For the sake of explaining the casting
procedure, assume today is Wednes-
day. The older segment was cast on
Monday and is now cured and ready for
the storage yard. The old segment was
cast yesterday or Tuesday and was
match cast against Monday's segment.
Today a new segment will be cast
against Tuesday's segment.
Fig. 2 shows the form arrangement
for short line match casting. The French
call these forms casting machines.
The appropriateness of this name will
soon become apparent for they really
are machines. The length of the side
forms is equal to the length of the seg-
ment being cast plus 1 or 2 in. (25 or 51
mm) to seal around the match cast
joint. The side forms have the capability
of being folded back away from the
segment to permit removal of the seg-
ment. This is done either with screw
jacks or hydraulic rams. The collapsible
inside formwork which forms the void of
the box girder rolls on rails to allow re-
moval of the form, enabling the segment
to be lifted vertically.
When the workers arrive at the pre-
casting yard on Wednesday morning,
the side forms are closed, the inside
form is rolled forward, Tuesday's seg-
ment is in the new segment position and
Monday's segment is in the old segment
position. The first operation is to deter-
mine the relative position as the seg-
ments actually were cast. This is done
by shooting elevations and centerline
with an accurate survey instrument.
These shots are called early morning
shots. (Geometry control will be dis-
cussed in greater detail later.) Once the
early morning shots are taken, the forms
are released and Monday's segment is
taken to the storage yard. Tuesday's
segment is then moved from the new
segment position to the old segment po-
sition.
All of the geometry of the bridge (hori-
zontal or vertical curves and super-
elevation or transitions) is cast in by ad-
justing the old segment. The forms are
never adjusted for geometry. Therefore,
once Tuesday's segment is in the old
segment position, its attitude is adjusted
by screw jacks betwee n the carriage and
the soffit to provide the proper bridge
geometry. A prefabricated reinforcing
bar cage is then set in the new segment
position. Once the side forms are closed
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Fig. 3. Casting machine. Proper geometry is obtained by adjusting the alignment of
the cast-against (concrete) segment with screw jacks under the supporting soffit.
and the inside form is rolled forward, the
casting machine is ready for casting to-
day's (Wednesday's) segment.
Fig. 3 shows a casting machine for a
short line match cast system. Note the
screw jacks for adjusting the attitude of
the old segment.
Fig. 4 is a schematic of the geometry
control layout for a short line casting
method. The survey instrument is
PERMANENT
PERMANENT
TARGET
INSTRUMENT
STEEL
BULKHEAD
W
SEGMENT
7/ 7 ,177/77/77/ 77777/77?/
PERMANENT
SOFFIT
Fig. 4. Geometry control layout for short line
system.
The instrument support and
target should be stationary and permanent.
PCI JOURNAUJuly-August 1980
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nent survey control points.
its are located over the
ninate any influence of top
rents. The cen ter points
9oretical centerline.
ally a theodilite capable of measuring
accurately to
32
of an inch (nearly 1
mm). The permanent target is generally
a concrete pile driven into the ground
and insulated to prevent bending due to
the sun shining on one side. Elevations
are shot using a surv ey rod. A m etric rod
may be more practical because the
smallest graduations are approximately
2
of an inch which eliminates some of
the guesswork involved when using rods
marked in feet and inches. The data
measured are elevation differences so
the metric rod does not result in exten-
sive unit conv ersion.
The segment survey control point po-
sitions are shown in Fig. 5. Each seg-
ment has six control points—four over
the two webs and two on the centerline.
Round-headed bolts placed 2 or 3 in.
(51 or 76 mm) from the edge of segment
are used for the elevation control points.
They are assumed to be at the edge of
the segment for computation purposes
and should always be placed over the
webs to eliminate any influence of top
slab deflections. Since only relative po-
sitions of segments are of concern,
these bolts do not have to be placed at
any specific elevation but may be placed
in the wet concrete so the bottom of the
head is approximately at the concrete
level. The early morning shots on these
points establish the basis of relative po-
sitions.
The two centerline control points usu-
ally consist of U-shaped wires placed in
the wet concrete after the top slab has
ERECTED CANTILEVER
FINAL PROFILE
THEORETICAL
CASTING
CURVE
PIER
LOCATION
A = DEFLECTION DUE TO PRESTRESS
Fig. 6. Theoretical casting curve is drawn from data provided by the design engineer.
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been finished. The theoretical centerline
is established by notching these wires
with a hammer and chisel during the
early morning shots.
The designer of the bridge will provide
information to develop a theoretical
casting curve. The theoretical casting
curve is a curve along which the seg-
ments should be cast so the final de-
sired alignment will be achieved after all
deformations. The computation of these
deformations is quite intricate since most
are time dependent and interdependent.
Thus, a good computer program is
needed for maximum accuracy. Among
the causes of deformations are self
weight of the structure, camber due to
prestress, prestress losses, creep and
shrinkage of the concrete and tempera-
ture variations.
For the sake of simplifying this discus-
sion, let us consider the deformation of
camber due to prestressing. Fig. 6
shows a crest vertical curve as a final
desired alignment. Assuming balanced
cantilever erection, the erected can-
tilever would deflect upward an amount
A due to the prestressing as represented
by the erected cantilever curve in Fig. 6.
Therefore, it is obvious the segments
must be cast with -a downw ard deflection
of A so when the camber occurs the
proper alignment will be achieved. A
curve depicting this dow nward de flection
is the theoretical casting curve. In reality,
when all the deformations are consid-
ered, the theoretical casting curve usu-
ally bends upward rather than down-
ward.
Fig. 7 (top) shows the theoretical
casting curve developed previously.
Since segments cannot be cast curved,
the curve is approximated by casting
segments on the chords. This is the pro-
cedure followed whether the curve is
horizontal or vertical. Therefore, chords
equal to the length of the segments are
laid out on the theoretical casting curve
so a tangent to the curve can be drawn
at the points of intersection of the
chords. Angles B, and B
can then be
measured from the local tangent defin-
ing the desired relative position of the
segments as they are match cast and
erected. This must then be related to the
position of the ca sting m achine.
B
THEORETICAL CASTING
CURVE
CASTING
8 ERECTI
ON
DIRE TION
ĤORIZONT̂ ^
VERTICAL Z
NEW SEGMENT
0
tnmininrnm
Fig. 7.
General
method for determining relative position of segments to obtain the
desired geometry.
PCI JOURNAUJuly-August 1980
71
B 2
R +R
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AS-CAST CURVE
JB, B2 C
Lt®
v
C
A
ST ING a ERECTION _
\ \
DIRECTION
HORIZONTAL
B1 BZ C
—
VERTICAL
NEW SEGMENT
OLD SEGMENT
2
O
177/77/77
77 7 rirrim
THEORETI L
CASTING CUR VE
Fig. 8. Segments must be adjusted to compensate for casting errors.
Two assumptions relative to the cast-
ing machines must be made—the first
can be controlled, the second cannot.
The first assumption is that the steel
bulkhead at the opposite end of the new
segment from the cast-against segment
is established and m aintained ab solutely
vertical with the top being completely
horizontal. In addition, the bottom soffit
is established and remains absolutely
horizontal. The second assumption is
that the segment being cast is perfect.
While this second assumption is not too
important to this explanation, it is very
significant when performing actual
geom etry control procedures.
To transfer the segment relationship
from Fig. 7 (top) to the casting machine,
one has to examine the direction of
casting and erection. In this case Seg-
ment 1 is cast and erected before Seg-
ment 2. Therefore, on the casting
ma chine Segm ent 1 is in the old position
and Segment 2 is in the new segment
position as shown by Fig. 7 (bottom).
Remembering the steel bulkhead lo-
cated on the left side of Fig. 7 (bottom)
is always vertical and the soffit is always
horizontal, one must adjust the attitude
of Segment 1 to duplicate the segment
relationship found in Fig. 7 (top). This is
done simply by rotating Segment 1 by
an angle equal to the summation of
B,
and
B2.
The procedure just described is
theoretical and idealized. Now let's get
practical. When we remember the seg-
ments weigh 40 to 50 tons or more and
the concrete is steam cured, raising the
temperature of the concrete and steel
casting machine to 150 F (65 C), things
are likely to move. In fact, they always
do
The purpose of the early morning
shots is to determine the magnitude and
direction of movement or casting error.
These data are plotted directly on the
theoretical casting curve as shown by
Point B on Fig. 8 (top). In this case, the
actual relationship between the two
segments previously cast results in the
end of Segment 1 being above the
theoretical casting curve. However, it
could just as well have been below it.
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Therefore, to get back to the theoretical
casting curve when casting Segment 2,
assumed to be perfect, a correction
must be included in the attitude of Seg-
ment 1 as it is placed in the old or cast-
against position. Th erefore, as shown by
Fig. 8 (top and bottom) the proper angle
of rotation of the segment is B, plus B2
plus a correction C.
T he curv e generated by plotting all the
early morning shot data is a curve which
wiggles on either side of the theoretical
casting curve. This curve is known as
the as-cast curve. If the as-cast curve
starts deviating away from the theoreti-
cal casting curve, the engineer knows he
has serious problems and can take cor-
rective steps before the situation gets
out of hand. The as-cast curve is also
valuable information for the field en-
gineer because it shows the actual re-
lationship between the segm ents as they
were cast. This relationship must be du-
plicated again when the segments are
erected.
It is strongly recommended that all of
the casting geometry control be set up
graphically and drawn to the largest
possible scale. This not only includes
the two previously mentioned curves but
the determination of rod readings to set
the proper attitude of the cast-against
segment. Also, a separate set of curves
should be used for each line of control
points even though two of them may be
theoretically symmetrical. Frequently,
the early morning data will not be sym-
metrical.
Of course, mathematical equations
can be set up to calculate settings for all
the points since all have a straight line
geom etrical relationship. H owev er, these
equations should only be used as an in-
dependent check of the graphics. It is
much more difficult to determine tenden-
cies and directions by examining sets of
numbers than by examining graphical
plots.
The short line system does offer some
advantages. For example, the space re-
quired for set up is minimal resulting in a
centralized operation. Any geometry de-
sired can be obtained by twisting the po-
sition of the cast-against segment. The
primary disadvantage of the method is
the accuracy at which the cast-against
segment must be set. Also, the casting
machine must be flexible enough to
conform to the twisted cast-against
segment but rigid enough to adequately
support the loads. This is particularly so
when casting segments for a superele-
va tion transition.
Long line system—An
alternative to
the previously discussed short line sys-
tem is the long line system. The system
is similar except that a continuous soffit
the length of a cantilever is built. Figs. 9
and 10 show such an example. All the
segments are cast in their correct rela-
tive position with the side forms moving
down the line as each segment is cast.
Geometry control is established by ad-
justing the side forms and soff it. Variable
depth structures may be cast by varying
the elevation of the soffit, i.e., curves are
cast by c urving the soffit.
A long line is easy to set up and to
maintain control of the segments as they
are cast. Also, the strength of the con-
crete is not as critical since the seg-
ments do not have to be moved im-
mediately.
When considering a long line system
several things must be taken into ac-
count. First of all, substantial space is
required. The minimum length of soffit
required is generally a little more than
one-half the longest span of the struc-
ture. The foundation must be strong and
relatively settlement free because the
segment weight to be supported can be
5 tons per lineal foot or more. A ny curing
and handling equipment must be mobile
since the side forms travel along the
soffit. The contractor must set up a
monitoring system and adjust the soffits
periodically to correct for any settlement.
Fig. 11 shows the long line casting
system used to cast the segments of the
Kentucky River Bridge near Frankfort,
Kentucky. This bridge was completed in
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OUTSIDE FORMWORK
INSIDE FORMWORK
ELEVATION
PLAN
Fig. 9. Schematic of long line casting system. Side
forms move along a permanent soffit to cast individual
segments.
ELEVATION
PLAN
Fig. 10. As segments reach desired concrete strength,
they can be removed to the storage yard. A second
cantilever may be started with the addition of a second
set of side forms.
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Fig. 11. Long line casting system used for Kentucky River Bridge, Frankfort,
Kentucky. Photo courtesy: Construction Products, Inc., Lafayette, Indiana.)
1979. Jack Kelly, manager of Construc-
tion Products, Inc., believes that this
system proved easier and was more ef-
f icient for the v ariable dep th segm ents.
Cast in Place Segm ents
Another alternative is to cast the seg-
ments in their final position on the
structure. Numerous projects have been
constructed in this manner across North
America. The bridge located near Vail,
Colorado (see Fig. 12) is one such
example.
Cast-in-place construction proves to
be very advantageous when large, very
heavy segments are encountered. In-
stead of handling the segments, only
materials have to be transported thus in-
fluencing the type and size of required
equipment.
The commonly used method for cast-
ing segments in place is with the use of
form travelers such as that shown in Fig.
12. Form travelers are moveable forms
supported by steel cantilever trusses
attached to previously completed seg-
Fig. 12. Form traveler used on a segmental project near Vail, Colorado. The usual
production rate for a form traveler is one segment every 3 to 5 days.
PC I JOUR NAL/July-August 1980
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8
7
6
5
4
3
2
2
3
4
56 7 8 9
IL POSSIBLE TEMPORARY
STRUT
Fig. 13. Balanced cantilever erection will probably be the most
commonly
used method for constructing segmental bridges. It
solves many problems
such as
environmental or existing traffic
constraints.
ments. The forms themselves may be
constructed of either wood or steel.
When balanced cantilever erection is
used, a minimum of two form travelers is
required.
The segment production rate for form
travelers is usually one segment every 5
days per traveler. Therefore, to ap-
proach the common precast segment
production rate previously discussed, at
least four travelers are required. The
5-day cycle time may be reduced with
concrete admixtures to increase the
early strength gain of concrete and by
the application of partial post-tensioning.
However, a practical minimum cycle
time is around 3 days per traveler.
Alignment variations and corrections
are more easily accommodated in
cast-in-place construction; but more cor-
rections will probably be necessary. The
increase in alignment corrections for
cast-in-place construction compared to
precast construction relates directly to
the age of the concrete when loaded.
Generally, the concrete is much younger
when loaded in cast-in-place construc-
tion.
The deformations due to creep and
shrinkage vary logarithmically with the
age of the concrete, with the values of
the deformations decreasing as the age
at loading increases. For instance, the
ultimate creep deformation of concrete
loaded at 7 days after casting will ap-
proach 1.5 times that for the same con-
crete when loaded 8
days after casting.
Also, one has to remember the 5-day
age difference for each segment results
in a significant difference in the creep
rate when an entire cantilever is
analyzed. (This complexity is a further
reason for using a computer in any
bridge job.)
Erection M ethods
alanced cantilever
—Balanced
an-
tilever erection, as shown in Fig. 13, is
quickly becoming the classic technique
when considering segmental construc-
tion. This method solves a multitude of
problems such as environmental restric-
tions, existing traffic problems, inacces-
sible terrain and man y others. It can also
be used readily with either precast or
cast-in-place segments. However, the
following discussion only relates to pre-
cast segments.
The general concept is to attach the
segments in an alternate manner at op-
posite ends of cantilevers supported by
piers. As the segments are attached the
moment to be carried at the pier in-
creases in a manner shown by Fig. 14.
The hatched area represents the change
in mom ent when attaching Seg ment
The compressive stresses in the bot-
tom of the concrete section at the pier
build up similar to the moment variation.
H owev er, the theoretical tensile stresses
occurring at the top of the same section
are offset by the post-tensioning forces
applied at a rate similar to the moment
increase. It is important to remember
that the top of the concrete section is
essentially operating at capacity during
the entire erection sequence. Therefore,
the construction loads must not increase
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Fig. 14. As the length of the cantilever grows, the magnitude of moment at
the pier increases. Since the post-tensioning tendons are also installed
and stressed
in increments
as segments are attached, the top concrete
stresses
are close to the design limits at all time s.
significantly over what has been as-
sumed in the design.
As segments are attached to the can-
tilever ends one at a time, an overturn-
ing moment is created and must be re-
sisted. This moment may be resisted by
post-tensioning the pier segment down
to the pier stem, providing temporary
supports on either side of the pier or
stabilizing the cantilevers with the erec-
tion equipment. The final choice belongs
to the contractor but the designer must
assume and detail a method for a stress
evaluation and parameters for the con-
tractor.
T he segments may be delivered to the
ends of the cantilevers by many means.
T he most economical and probably m ost
commonly used method in the United
States is lifting the segments with
cranes. Crane erection will probably be
more common in the United States than
has been experienced in Europe be-
cause of the greater available capacity.
Only restrictions which limit crane mobil-
ity make other methods more attractive.
F ig. 15 shows segm ents being lifted by a
barge crane.
Fig. 15. Crane erection is probably most
economical in the United States due to
crane
availability.
Access to the area
under the bridge must be available.
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Fig. 16. The launching gantry eliminates the
need
for construction
access
beneath
the structure. The gantry shown in this photo is in the
process
of moving to the
next
span to start a new cantilever.
Fig. 16 shows a launching gantry
placing segments for balanced can-
tilever erection. The launching gantry,
developed by Jean Muller in France, is a
machine capable of transporting a seg-
ment from a completed portion of the
bridge or from below the bridge to either
end of the cantilevers being erected.
The first project in the United States to
be erected with a launching gantry is the
Kishwaukee R iver Bridge near R ockford,
Illinois (see Fig. 17).
Launching gantries come in all sizes
and shapes. They can vary from the
large one shown in Fig. 16 to the small
simple one shown in Fig. 18. Both serve
identical functions; only the size of the
spans and segments change. Most
launching gantries have the capability to
move themselves once a cantilever is
completed and another is ready to
Fig. 17. This launching gantry is being
used to
erect the Kishwaukee River
Bridge near Rockford, Illinois. This is
the first such use of a launching gantry
in the United States.
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CART FOR
SEGMENT TR4
N
PMENT
0)
Fig.
20. A schematic of the progressive placing erection system. The system may
prove valuable when the
area is restricted
for substructure construction such
as the
Linn Cove Viaduct in North Carolina see Fig. 21 on opposite page).
begin. They may be equipped with two
lifting devices enabling simultaneous
attachment of segments minimizing re-
quired overturning moment provisions.
The various details of the launching
gantry depend on the size of structure
and the economics involved.
Launching gantries are particularly
advantageous when accessibility to the
area beneath the structure is restricted
by environmental consideration. By de-
livering segments across previously
completed portions of the bridge, access
to the area beneath is not required ex-
cept to build the substructure. New
bridges can be erected over existing
traffic and/or buildings with minimal dis-
turbance. This is a tremendous con-
struction advantage in urban areas.
Launching gantries can be used to erect
curved bridges as well as straight ones.
Accessibility to the structure is generally
the ov erriding co nsideration for balanced
cantilever erection with a launching
gantry.
Gantry cranes similar to the one
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Fig. 21. A computer generated photo of the Linn Cove Viaduct. The bridge is being
built from the top (including foundations) by the progressive placing
concept.
Photo courtesy: Figg and Muller Engineers, Inc.)
shown in Fig. 19 may also be used to lift
the segments. The gantry crane,
whether m ounted on rubber tires or rails,
travels between the ends of the can-
tilevers to lift the segments. Obviously,
the practicality of this type of equipment
is limited to low level structures over
land such as viaducts. But gantry cranes
possess faster movement than track
mounted cranes and may eliminate the
need for a second large capacity crane
on a project.
Progressive placing—
Progressive
placing is a modification of the balanced
cantilever concept. Fig. 20 shows the
basic concepts of progressive placing.
Instead of starting erection at a pier and
proceeding in two directions, progres-
sive placing erects cantilevers in only
one d irection.
The equipment required is a crane
capable of lifting a segment delivered
along the previously completed portion
of the bridge and swinging around and
lowering the segment to be attached to
the end of the cantilever. The crane
shown in Fig. 20 (top) is a swivel crane
available in Europe. A stiff leg derrick
may also be used.
As the cantilever extenas in one di-
rection, the capacity of the section lo-
cated at the pier is soon exceeded.
Therefore, a temporary support must be
provided to prevent overstress. The
method shown in Fig. 20 is a system of
temporary cable stays which are moved
from pier to pier as construction pro-
ceeds.
As shown in Fig. 20, hydraulic jacks
can be attached to the stays to control
the stay stresses and orientation of the
cantilever. An alternate and maybe sim-
pler method is to provide jacks beneath
the legs of the vertical steel tower. Thus,
the stress in the stays can be varied by
raising or lowering the steel tower. The
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Fig. 22. Steel launching nose used to control erection stresses for the incremental
launching concept.
As
the bridge moves across the supports, the concrete is
subjected to both maximum positive and negative dead load moments.
primary advantage here is having only
two jacks to control the operation.
Instead of the temporary cable stay
system, a system of temporary bents
may be provided beneath the structure.
If permitted by the terrain, temporary
bents may be a more economical and
faster solution. Of course, each project
must be evaluated separately.
Progressive placing not only provides
an opportunity to construct the
superstructure unhindered by obstacles
but provides the ability to also build the
piers from the top. A case in point is the
Linn Cove Viaduct located near Grand-
father Mountain in North Carolina. This
project, representing the final link of the
Blue Ridge Parkway, will take the park-
way around a mountain in a scenic and
environmentally sensitive area. In fact,
the terrain is so rough, it is impossible to
get heavy construction equipment to the
pier sites without extensive damage.
The National Park Service, owner of the
bridge, stipulated a construction road
could only extend from the abutment to
the first pier. Fig. 21 shows a computer
generated image of the completed proj-
ect. Co nstruction was started in 1979.
The Linn Cove viaduct is being
erected by the progressive placing
technique with temporary bents located
at midspan between the permanent
piers. Both the bents and piers are being
constructed from the top with the stiff leg
derrick used to place the precast seg-
ments. As the end of the cantilever ap-
proaches a pier location, the derrick
lowers men and equipment to drill and
cast 9-in. microshaft piles. The elliptical
shaped footing is then cast w ith concrete
delivered over the completed portion of
the bridge and lowered with the derrick.
The pier stems consist of precast seg-
ments delivered and placed in a similar
manner. Once the vertical pier post-ten-
sioning tendons are installed and
stressed, the superstructure segment
placing resumes until the next pier loca-
tion is reached. Then the process is re-
peated.
Incremental launching
—Incremental
launching is a technique where seg-
me nts are cast at the end of the crossing
and pushed across by large hydraulic
rams. This method is most useful when
the piers can be easily located at regular
intervals.
Temporary support bents may or may
not be required at midspans depending
on the span length. A steel launching
nose is generally attached to the end of
the segments, as shown by Fig. 22, to
control erection stresses. The segments
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are usually 50 to 100 ft (15 to 30 m) in
length.
Incremental launching is best adopted
to bridge lengths of 1000 to 2000 ft (305
to 610 m) unless other considerations
are involved. For instance, when the
working area is severely restricted,
bridges up to 4000 ft (1220 m) in length
may be achieved by launching from both
ends.
When considering incremental
launching, one must consider some re-
strictions. The horizontal and vertical
alignment must either be straight or of a
constant radius of curvature; preferably
250 ft (76 m) or greater. In addition, the
top slab must have a constant crown or
constant superelevation without any
transitions.
The Wabash River Bridge near
Covington, Indiana, was the first incre-
mentally launched bridge in the United
States. This project (see Fig. 23) was
completed in 1977. The original design
plans and specifications were based on
precast segmental construction erected
by balanced cantilevers with cranes. The
contractor exercised an option to alter
the construction method with a reported
sav ings to the State of Indiana.
It is believed precast segments may
also be launched although the author is
not presently aware of any projects
using this method. Identical restrictions
would probably apply but the basic prin-
ciples could be used to launch precast
segm ents to achieve a desired result.
Span by Span
—Span by span erec-
tion may be the most economical
technique for erecting seg me ntal bridges
in the medium span range [less than
250 ft (76 m)]. This method utilizes an
assembly truss spanning between per-
manent piers to support precast seg-
ments prior to installation and stressing
of post-tensioning tendons. Segments
are placed on the assembly truss by a
crane in approximately their final posi-
tion. After all segments comprising a
span are assem bled, the post-tensioning
Fig. 23. The Wabash River Bridge near Covington, Indiana, was the first
incrementally launched bridge in the United States. This photo shows the bridge
approaching the opposite side of the river.
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Fig. 24. The span by
span erecti
on
scheme has proven to be very economical for
shorter span segmental bridges. The method is applicable to both
precast
and
cast-in-place segmental
construction. (Courtesy: Figg Muller Engineers, Inc.)
tendons are installed and stressed. Fig.
24 show s the basic system.
Jean Muller developed the span by
span concept in an effort to Americanize
segmental construction. The basic ob-
jectives were to simplify the system
thereby reducing the number of opera-
tions required. Reduced labor require-
ments are not a significant factor be-
cause segmental construction in general
is not labor intensive.
Span by span techniques allow addi-
tional modifications to the components
of the structure. Primarily, the post-ten-
sioning tendons may all be continuous
for the total span length and may be lo-
cated in a draped manner providing
most efficient use of post-tensioning
forces. Also, only one operation of in-
stalling and stressing tendons is re-
quired per span.
The Long Key Bridge in Florida is the
first structure to be erected in this man-
ner. The spans are 118 ft (33 m) in
length. Fig. 25 shows the assembling of
segments for one of the spans. The
contractor has readily achieved an erec-
tion rate of three spans per week result-
ing in the essentially complete construc-
tion of 354 ft (108 m) of superstructure
per week. Only the casting of the barrier
curbs remains to com plete the structure.
The span by span erection technique
allowed two other modifications of nor-
mal segmental construction procedures.
The Long Key Bridge is the first precast
segmental bridge to be constructed with
dry joints. Normal practice is to seal the
joints with epoxy. However, dry joints
are not recommended for bridges which
may be subject to freeze-thaw condi-
tions and deicing chemicals. Also the
post-tensioning tendons are located in
the void of the box girder as opposed to
locating the tendons in the concrete
walls of the sections. The tendons are
protected with plastic conduits and
grout. This tendon location simplifies the
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ERE TED
CLOSURE
SEGMENT _ J01NT
y_SEGMENT ASSEMBLY
PIER
ASSEMBLY TRUSS
Fig. 25. Assembly truss being
used to erect the Long Key Bridge in Florida. This
photo shows the truss being installed between two piers. The contractor erected
three 118-ft
spans per
week. (Courtesy: Figg and Muller Engineers, Inc.)
casting of the segments and eliminates
any problems of tendon alignment at the
segm ent joints.
The Seven Mile Bridge located near
the Long Key Bridge in the Florida Keys
(see Fig. 26) will also be erected span
by span but the contractor has elected to
assemble the segments on a barge and
lift the entire 135 ft (41 m) span at one
time. A temporary post-tensioning sys-
tem and support frame will hold the
segments together during the lift. The
contractor hopes this modification will
provide an even faster construction rate
than achieved at Long Key.
Co ncluding Rem arks
The bottom line when choosing a con-
struction technique for a particular seg-
mental bridge is economics—and cor-
rectly so. The prospective contractor
should thoroughly investigate all sys-
tems complying with specified parame-
ters and choose the one which provides
the least cost. The casting method,
Fig. 26. Rendering
of Seven
Mile Bridge. (Courtesy: Figg Muller Engineers, Inc.)
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post-tensioning system or erection pro-
cedure cannot be evaluated separately
for each is only a part of the entire con-
struction technique to be used. All as-
pects of the project have to be consid-
ered together.
The design engineer should evaluate
the parameters of the project and base
the design on the most economical
method. However, all other methods
complying with the parameters should
be allowed as contractor's options.
There is essentially no difference in the
final quality of the project.
Many different construction tech-
niques have been discussed in this
paper. H owev er, it should not be inferred
that these are the only techniques avail-
able. Time and space permitted the in-
clusion of only some of the most com-
mon construction methods. Engineers,
whether con tractors or designers, should
use their ingenuity to use the common
techniques and develop new techniques
to reduce c onstruction costs.
Geometry control techniques were
discussed more extensively for short line
match casting than for the other
methods. This is because this geometry
control is probably the most difficult to
understand. With proper understanding
and attention to details, the results will
be excellent. For instance, the Long Key
Bridge is being cast by the short line
me thod and w ill not receive an ad ditional
wearing surface. The author has ridden
on the completed portion of the bridge in
a vehicle traveling at various highway
speeds. The segment joints could
neither be felt nor heard. This is a tribute
to what can be accomplished.
Some contractors have expressed
concern over the additional time and
money required to evaluate the options
before bidding projects. The days in
which design engineers could tell con-
tractors exactly what to do are quickly
coming to an end. To create the most
economical projects, competition be-
tween materials and the methods of
using the materials must be enc ouraged.
We in North America are not going to
the design-build concepts prevalent in
Europe, but are going to approach that
idea and probably be somewhere in
between. This is also a tendency in
Europe.
In the future, projects and construction
methods are going to be more en-
gineering oriented requiring a coopera-
tive effort between designers and con-
tractors with a required increase in con-
tractor eng ineering staffs. A lso, there will
be more competition between construc-
tion materials including the availability of
two complete sets of plans based on
different materials. The final result
should be a much more economical use
of construction resources.
SELECTED REFERENCES
1.
Muller, Jean, Ten Years of Experience in
Precast Segmental Construction, PCI
JOURNAL, V. 20, No. 1, January-February
1975, pp. 28-61.
2.
CE B/FIP R ecommendations for the De-
sign and Construction of Concrete Struc-
tures
Third Edition, Cement and Concrete
A ssociation, London, England, 1978.
3.
PCI Committee on Segmental Construc-
tion, Recommended Practice for Seg-
mental Construction in Prestressed Con-
crete, PCI JOURNAL, V. 20, No. 2,
March-A pril 1975, pp. 22-41.
4.
Precast Segmental Box Girder Bridge
Manual
Joint Venture: Prestressed Con-
crete Institute, Chicago, Illinois; Post-Ten-
sioning Institute, Phoenix, Arizona, 1979.
5.
Post-Tensioned Box Girder Bridge Man-
ual,
Post-Tensioning Institute, Phoenix,
Arizona, 1978.